This invention relates to friction members and particularly to nonmetallic, heat resistant, disc-type friction members for brakes or clutches.
Difficulties can often arise with brakes and clutches in high energy situations. For example, problems can arise in high energy brakes such as those used in aircraft, where the brake device must absorb great quantities of heat and withstand extremely high temperatures during the braking operation. When the engaging surfaces of the brake become overheated, decreases in the coefficients of friction can result. Overheating can also cause distortion of the brake which causes rapid wear. One way to alleviate these problems is by the use of "heat sink" materials within the brake which draw heat away from the engaging surfaces and the surrounding structures.
Generally, heat sink elements are heavy and massive. Often times, as in aircraft brakes, when it is desirable to reduce weight and/or mass, brakes are expected to accommodate the heat created during braking operations, provide an effective and sustained braking action, and be lightweight and compact.
Steel has been used as a heat sink material, but its weight and mass are disadvantages when lightweight and compact brakes are required. Lighter weight metals (such as beryllium) with heat absorption characteristics similar to steel have been used as heat sink materials where weight is critical, but the cost of such metals is a disadvantage.
Nonmetallic brake materials have been proposed to solve heat problems in situations where weight and mass are critical. Carbon cloth laminate discs of the type disclosed in U.S. Pat. No. 3,730,320, assigned to the present assignee, have been proposed, but these are still considered by many to be expensive from both material and processing standpoints. Other carbon cloth laminate discs are disclosed in U.S. Pat. Nos. 3,552,533 and 3,548,979.
Relatively inexpensive forms of lightweight bulk graphite have been proposed as a brake friction member. However, problems can arise when bulk graphite is used by itself as a friction member due to its brittleness and impact strength which are insufficient for high energy applications. Furthermore, bulk graphite does not have sufficient friction characteristics for an adequate friction surface.
Composites of heat sink materials and friction materials have also been proposed. Some friction materials resist deterioration and wear under the most severe operating temperatures when used in combination with a heat sink element. For example, carbon cloth laminate materials have been used as a friction face by mounting them on some type of heat sink core material, such as metal in U.S. Pat. No. 3,306,401. A satisfactory bond between two such materials is difficult due to the different coefficients of thermal expansion of the materials, which causes separation of the materials upon extreme heating. Fabrication of such composites is thus difficult because the lining is typically mounted in small segments by rivets, clamps, or other mechanical means to avoid problems of thermal expansion in the lining and the core.